1. Field of the Invention
The present invention relates to a scintillation counter for detecting radiations such as .beta. rays. More particularly, the present invention relates to the scintillation counter being for in vitro or in vivo detection of .beta. rays or being for detecting .beta. rays from radioactive contamination in small diameter pipes at, for example, nuclear power plants.
2. Description of the Related Art
Japanese Patent Application Publication Kokai No. HEI2-206786 describes the scintillation counter 50 shown in FIG. 1 (a) for in vivo detection of .beta. rays emitted from an organism injected with a radioactive substance. .beta. rays include electrons (.beta..sup.-) and positrons (.beta..sup.+).
As shown in FIG. 1 (a), the scintillation counter 50 includes a scintillation fiber 51 and an optical detector 52 joined by an optical fiber 53.
To measure the concentration of radiation in a certain area of an object to be measured, the scintillation fiber 51 is inserted as a probe into the area. However, the scintillation fiber 51 also picks up .gamma. rays from other sources, which are then converted into a signal. The signal appears as a background noise which lowers the precision of the .beta. ray measurement.
Japanese Patent Application Publication Kokai No. HEI4-274792 describes a scintillation counter 60 shown in FIG. 1 (b) for detecting the distribution of radioactive substances in the internal wall of a pipe. The scintillation probe 61 of the scintillation counter 60 has a bundle of a plurality of scintillation fibers. The bundle consists of two groups of parallel scintillation fibers: a measurement fiber group 61A and a reference fiber group 61B. The measurement fiber group 61A surrounds the reference fiber group 61B. The reference fiber group 61B consists of a plurality of parallel scintillation fibers 161B each of which is covered with a shield material 64 for blocking out .beta. rays, as shown in FIG. 1(c). Therefore, the scintillation fibers 161B of the reference fiber group 61B are sensitive to only .gamma. rays. To the contrary, the measurement fiber group 61A consists of a plurality of parallel scintillation fibers 161A each of which is not covered with such a shield material 64. Accordingly, the scintillation fibers 161A of the measurement fiber group 61A are sensitive to both .beta. rays and .gamma. rays, similarly as the scintillation fiber 51 of FIG.. 1(a). A plurality of optical fibers 63A connect the plural scintillation fibers 161A to an optical detector 62A. Similarly, a plurality of optical fibers 63B connect the plural scintillation fibers 161B to an optical detector 62B. The fiber bundle 61 can be inserted as a probe into an object to be measured. The intensity of .beta. rays only can be measured by subtracting the value detected by the optical detector 62B from the value detected by the optical detector 62A.
This publication also describes the scintillation counter 70 shown in FIG. 1 (d) wherein a plurality of scintillation fibers 171A of a measurement fiber group 71A are paired with a plurality of scintillation fibers 171B (each covered with a .beta.-ray shield material 64) of the reference fiber group 71B. A scintillation probe 71 is formed by attaching the pairs of scintillation fibers 171A and 171B to the outer surface of a cylindrical holder 75 at appropriate intervals in the circumferential direction.
Each of the probes 61 and 71 of the above-described scintillation counters is thus formed from a bundle of a plurality of parallel scintillation fibers for detecting the distribution of radiation. Accordingly, the probes 61 and 71 become too thick to be inserted into pipes with small diameters or living objects, particularly small animals.